1. Field of the Invention
The present invention relates to a digital line interface section in a private branch exchange system connected to a digital network and, more particularly, to a private branch exchange system having a specific information transmitting function by means of which information indicating the occurrence of a failure between a digital line interface section and a digital line can be transmitted.
2. Description of the Related Art
In a conventional digital private branch exchange system, a private branch exchange is connected to a digital network via a digital line interface including a digital trunk circuit and a channel service unit.
When speech data input from an extension terminal is to be transmitted from an extension interface circuit to a digital trunk circuit, the speech data output from the extension interface circuit is PCM-converted, switched from an input to output highway by a time division switch, and transmitted to the digital trunk circuit. Transmission of the speech data is repeatedly performed for each sync signal having a frequency of 8 kHz. Thereafter, the speech data is transmitted onto the digital line in a multiframe manner via the digital trunk circuit and the channel service unit.
Reception data of a multiframe format from the digital line via the channel service unit is converted into data synchronous with a 2.048-MHz clock signal by the digital trunk circuit. The speech data in the converted data is switched from the input to output highway by the time division switch and transmitted to the extension terminal via the extension interface circuit.
When communication is to be performed by using the digital line as described above, a failure such as step-out may occur between the channel service unit and the digital line. In the conventional private branch exchange system, however, specific information cannot be generated to alarm occurrence of a failure to the terminal. Therefore, the failure cannot be detected until communication actually starts. That is, when a failure occurs during communication, not only the communication is interrupted, but also the type and location of the failure cannot be recognized from the extension terminal, resulting in poor service.